Recently, liquid crystal display devices and other flat-panel display devices are used in various fields as image display devices for personal computers, word processor-dedicated machines, television set or the like as well as for a projector display device; in view of their small depth dimension and small weight as well as small electric power consumption.
Active-matrix liquid crystal display (active-matrix LCD) devices in particular, which has pixel-switching elements arranged on each display pixel electrode, enables to achieve good image quality without crosstalk between adjacent pixels. Because of these features, active-matrix LCD devices are being earnestly investigated and developed.
In following, a light transmissive one of the active-matrix LCD devices is exemplified for explaining its construction.
An active-matrix LCD device is comprised of a matrix array substrate (hereinafter referred as array substrate) and a counter substrate, which are closely opposed to each other with a predetermined gap, and of a liquid crystal layer held in the gap.
The array substrate has signal lines and scanning lines, which are arranged in a latticework on an insulator substrate such as a glass plate, and are overlapped to sandwich an insulator film. On each rectangular patch defined by the signal and scanning lines, a pixel electrode is disposed and formed of a transparent electro-conductive material such as Indium-doped tin oxide (ITO). At around each crossing of the signal and scanning lines, a pixel-switching element is disposed for controlling a respective pixel electrode. When the pixel-switching element is a thin film transistor (TFT), gate and signal electrodes of the TFT are respectively connected with scanning and signal lines; and a source electrode of the TFT is connected with a pixel electrode.
The counter substrate has a counter electrode formed of a transparent electro-conductive material such as indium-doped tin oxide (ITO), on an insulator substrate such as a glass plate. When to realize color display, color filter layers are formed on the array or counter substrate.
Conventionally, as to achieve uniform and constant thickness of the liquid-crystal layer, spherical resin beads of uniform diameter are sprayed on either of the array and counter substrates as spacers, before attaching the substrates with each other. This technique of spraying the spherical beads or spacers has following drawbacks. (1) Spherical spacers are partly agglomerated; and in severe occasions, defects of luminous dot appear on screen of the display. (2) Due to uneven distribution of the spherical spacers, an area scarce of the spacers may arise and become smaller in thickness dimension of the liquid-crystal layer compared to other areas; and in such occasion, unevenness of contrast ratio within the display screen will be arisen as to deteriorate quality of image displaying. (3) The spherical spacers are distributed even in display regions each within a pixel dot. This causes “light leakage”, which means light passing through the liquid-crystal layer irrespective of voltage applied to the layer; through the spherical spacers by themselves; and through portions around the spherical spacers, at which orientation of liquid-crystal molecules is deteriorated by the spacers. (4) A designed gap and/or stress distribution between the substrates are made uneven by the spherical spacers put on raised parts (such as TFTs) and those put on the other parts on the array substrate.
Hence, there has been also adopted a technique of forming projections or pillar spacers on either of the substrates by film formation and subsequent patterning. Please see JP-2000-029055A, that is, Japan's Kokai or patent publication of the application No. 2000-029055. By such technique, the spacers are placed surely on intended position. Each of the pillar spacer may be arranged in an area fairly smaller than a storage-capacity area within a pixel dot; thus, even when the pillar spacers are formed on the counter substrate, alignment error between the substrates may be absorbed by such area-wise margin.
Nevertheless, when projection-wise dimension of the pillar spacers is large, it is rather difficult to achieve sufficient curing of the basal parts of the pillar spacers; and thus, some trouble arises in a manufacturing process. Moreover, when brushing is made for cleaning, portions of the pillar spacers are chipped off or scraped away. Because of such chipping or scraping along with the insufficient curing of the basal parts, troublesome unevenness arises in thickness of the liquid crystal layer.
Meanwhile, investigation on ink-jet technique has been made as to arrange the spherical spacers in predetermined positions on the substrate. Please see JP-2004-145101A. A nozzle head having a row of nozzle apertures is intermittently moved by a predetermined interval while emitting dispersion medium or liquid containing the spherical spacers. When the dispersion medium is evaporated, the spherical spacers are disposed in the predetermined positions on the storage-capacity forming area or the like. Nevertheless, the spherical spacers are occasionally shifted and deviated from the predetermined positions, on course of placing the spacers by the ink-jet technique, or after the placing of the spacers. Adhesives have been occasionally used for fixing the spherical spacers; nevertheless, the spacers sometimes are shifted or migrate before curing of the adhesive.
In otherwise, JP-1999 (H11)-65479A proposes a following method. The spherical spacers of small and larger diameters are arranged on the substrate by use of the ink-jet technique in order that the small ones are placed on the electrodes and the larger ones are placed on other areas. By this method, the gap between the substrates may be curbed to be fluctuated between areas of the electrodes that are somewhat raised from surroundings and the other areas. Nevertheless, the method of JP-1999-65479A does not completely eliminate troubles that may be caused by such as partial deviation of the spherical spacers. In particular, the method of JP-1999-65479A is hardly applicable when area-wise dimensions of the electrodes are small. In general, the method is practically applicable only to the LCD devices of STN (non-active simple matrix), we believe.
In view of the above drawbacks of using the spherical spacers in the LCD devices and its manufacturing method, it is aimed to curb positional deviation of the spherical spacers and curb unevenness in gap and/or stress between the substrates.